Motion detectors are generally utilized to sense the movement of objects to which they are attached. A common application of a motion detector is its use to detect motion of an automobile in an automobile alarm. The motion detector and its corresponding detection and control circuitry are colocated, typically in a metal or plastic housing. The housing is connected via screws, or other equivalent fasteners, to the automobile's body. When the alarm is activated, movement of the automobile, as detected by the motion detector, results in an audible alarm, flashing of the automobile's headlights and taillights, or remote actuation of the automobile owner's alarm pager depending on the functionality of the alarm's detection and control circuitry.
Motion detectors are fabricated using a variety of materials. Three of the most common motion detectors are crushed carbon sensors, micromachined sensors, and mercury-switched sensors. Crushed carbon sensors typically comprise a volume of encapsulated crushed carbon with an electrode attached to each of two opposing sides of the volume. The electrodes measure the resistance of the encapsulated crushed carbon. Thus, when the crushed carbon sensor is in motion, the crushed carbon moves and creates a resistance variation between the two electrodes. However, due to its material composition and sensor configuration, the crushed carbon sensor only detects motion in one dimension and has limited sensitivity. That is, the crushed carbon sensor is only sensitive to high acceleration and deceleration forces (greater than 1 G).
Micromachined sensors typically comprise a micromachined metallic object positioned within a recessed area of a silicon substrate. The recessed area includes four walls, each having an accelerometer electrode attached thereto. When the micromachined sensor is in motion, the metallic object moves within the recessed area and contacts the electrodes. Due to its mechanical and material construction, the micromachined sensor accurately senses low acceleration and deceleration forces (less than 0.1 G). However, the micromachined sensor only detects motion in two dimensions and cannot structurally withstand extremely high acceleration and deceleration forces (greater than 1000 G).
Mercury-switched sensors typically comprise a mercury-filled tube with electrodes at each truncated end and an electrical contact along the tube's body length. The electrical contact is isolated from the electrodes and typically connected to direct current (DC) ground. When the mercury-switched sensor is in motion, the mercury moves within the tube and provides contact between the electrical contact and either one of the two electrodes, effectively creating a DC ground at the contacted electrode. Although the mercury-switched sensor accurately detects low acceleration and deceleration forces, it only senses motion in one dimension, cannot endure extremely high acceleration and deceleration forces, is large in size (approximately 1.25 centimeters in length), and contains environmentally hazardous material (mercury).
Therefore, a need exists for a motion sensing apparatus that is small in size, that is environmentally preferable, that detects low acceleration and deceleration forces in three dimensions, and that can structurally endure extremely high acceleration and deceleration forces.